There's no way my little sister is dating some guy!
by randomfanfics98
Summary: Rin found out Gou went on a date with a certain.. someone. Being the protective older brother, he sets out to give that someone a piece of his mind. Hilarity ensues. One-shot


It was an ordinary Monday afternoon in the Samezuka dorms. Or at least it was _supposed_ to be. Swim practice had been cancelled due to "pool cleaning maintenance work". Grumpily, Rin went back to his room. Not interested in doing his homework, he opted to laze around on his bed instead, staring blankly at the ceiling. His phone was broken when Momo had thrown it into the pool as a bet, he was not happy about that. Apparently the redhead made a bet with another member of the swim team that he could piss Rin off and get away with it. He lost said bet. And now Rin was bored out of his mind, nothing to do since Sousuke was using the shared computer in his room.

"Yo, Rin." Sousuke called over, trying to get his attention. Rin turned over to face the other boy, not wanting to get off the bed. He just wanted to laze around today.

"What is it?" he grumbled sleepily. He was too tired to do anything. It was Monday, after all.

"Come check this out. Gou posted something online."

The mention of his little sister had Rin up in a flash. No incest. He dragged his feet over to the computer screen. Sousuke was on some website called Facebook. It was apparently some form of social media. For the life of him, he could not figure out how that thing worked, and just gave up after 3 days of trying. Gou, on the other hand, was more successful. She was an active user, constantly posting pictures of muscles and stuff, occasionally about the Iwatobi Swim Club. This one particular post, however, which Sousuke had drawn his attention to, was causing his eyes to bulge out. The post was dated Saturday, around 9pm. He crashed at Sousuke's place that night, meaning he didn't get to see her that day. The post read:

" _Thank you for the date today! I enjoyed it so much! The movie was pretty interesting too… I can't get enough of those muscles! 3333"_

"! WHAT?!" to say Rin was furious was an understatement. His whole face was red, Sousuke could tell he was probably mentally writing down a list of suspects to chase down for dating his little sister. Without another word, Rin marched out the door, grabbing his jacket on the way. Chuckling slightly, Sousuke scrolled down to view the picture that came with the post. If only Rin had stayed for a little longer, it would have saved him so much effort in searching…

The obvious first suspect was a certain Mikoshiba. More specifically, Momotarou Mikoshiba. It was no secret that he was head over heels for Gou. His muscles were something to look at too, for someone his age. Like, how can a 15 year old boy have the muscles of a 19 year old? Rin was in front of said boy's room door in less than a minute, impatiently banging on the door. When no one answered after, say, 2 seconds, he forced it open with a loud thrust.

"MOMO!" said boy was hanging upside-down off the top bunk, for god knows what reason. The sudden intrusion had caused his feet to lose his balance, sending him crashing to the ground head-first.

"Ow! What was that for, Rin-senpai?!" he shouted back, rubbing his head in pain.

"I didn't do anything. You fell yourself." Rin towered over the younger boy, staring down at the figure lying on the floor.

"What did you come here for, Rin-senpai?" Nitori was on his own bed, watching the scene unfold before him. He didn't seem at all fazed by Momo's ridiculous antics.

"Ah right. Back to the point." Cracking his knuckles, Rin picked Momo off the floor, slamming him into the nearest wall. Gripping the peach boy's shirt tightly, he was about to strangle him. With his free hand, he raised a threatening fist.

It was safe to say Momo was now terrified. Behind them was a shaking Nitori, bubbling with fear, not daring to step into whatever conflict Momo had gotten himself into this time. Maybe Rin was here to pay Momo back for ruining his phone.

"WHERE DID YOU GO ON SATURDAY?!" it was a seemingly normal question, but in this situation, Rin was yelling as if it were a life or death question. His overprotective brother side was finally showing.

"I went to town, Rin-senpai…." Momo muttered sheepishly, fear covering his eyes, blocking any form of rational thought, if he had any to begin with. "I went to the bulk store with Nitori-senpai!" he brightened up, looking at his smaller senpai hiding behind the bed.

"….Huh?" Rin was legitimately confused. What would Momo possibly need from the bulk store? It was more like a factory, where you could get 50 of something for the price of say, one. Hell, you needed a _member's card_ to shop there. Releasing his grip on Momo, Rin took a better look around the room. Indeed, there were some new items strewn about. In the corner, there were 10 boxes of corn flakes cereal stacked high, it looked like it would fall any moment. On the table, there were pens of all colours and shapes strewn about, and Momo had apparently tried to make a pen tower.

"Rin-senpai do you want to see my new water gun from the 100¥ store?!" Momo was already holding said water gun, pointing it straight into Rin's face.

"No." with that tone, he could probably rival Haru for flatness of a voice. He was in no mood to deal with Momo's nonsense today. He had more important things to do. Leaving Momo in the somewhat-capable hands of Aiichiro Nitori, he marched out the door, slamming it behind him. Onto the next suspect. A rather unlikely one, but better safe than sorry. He would check on everyone on his list, regardless of how unlikely it would be.

He cursed everything today. Why was the sun so damned hot? Why did the bus break down, making him walk the rest of the way? Why was he even doing this in the first place? There was only one thing he was thankful for by the time he got to his destination. The fact that his suspect never locked his front door, ever. He learned the first time he came over, when he wasted 5 minutes standing outside waiting for Haru to come out. He had given up and marched straight in, finding the front door unlocked, much to his annoyance. Now, he was standing in the middle of the living room, his target nowhere in sight.

"HARU!" he screamed into the house, hoping said boy would hear and come out from whatever hole he was hiding in. …No response. Sighing, he decided to poke around the house instead, maybe find something that might be helpful to his little 'search'.

He had searched the entire first floor, there was no sign of life anywhere. The only indication that someone was even living in the house was the laundry room. He had gone in there, and found Haru's boxers and swimsuits hanging on the laundry rack to dry. Haru had boxers with fish on them. Go figure.

He was about to leave the house, when he heard a running of water from upstairs. How had he not thought to look in the bathroom? He ran up the stairs, banging on the door once he found what he thought was the bathroom. He could hear sloshing of water from a tub from the inside. He was at the right place.

"Makoto?" Haru called, but apparently didn't make a move to get out of the tub.

Rin just got more pissed. That Haru was too reliant on Makoto! Thrusting the door open, he marched into the bathroom, paying no heed to Haru's shocked face.

"Rin…? What are you…?" Haru was cut off as Rin grabbed his shoulders, yanking him up and out of the tub. Thank god he was wearing his swimsuit this time.

He dumped Haru on the floor, right in the middle of the hallway, still dripping wet. Towering over him so he was trapped between him and the wall, Rin glared down with menacing eyes. "Haru! Where did you go on Saturday?" simple question, straight to the point.

"…Huh? Why did you come all the way here to ask me that?" Haru's voice was flat as ever, but his eyes gave away some sort of emotion. Shock? Absurdity? Whatever. Rin didn't care.

"JUST ANSWER THE QUESTION!"

"Fine… I went to the city." Haru was shaking his head, flicking water droplets all over Rin.

"… For what?" Rin was glowering. This was NOT the time he wanted to deal with Haru's crappy attitude. He could act like an ass any other day, just not today.

"Would you believe if I said I went out with my girlfriend?" that's it. Now Haru's just trying to mess with him. Not doing a good job at that.

"NO." flashing the trademark Matsuoka glare, Rin drew closer to Haru, menacing gaze in his eyes. "Tell me the truth, Haru. Seriously." Rin's voice had calmed down to a sort of pleading, he didn't want to sound like he was a needy child. But really, his anxious older brother side was taking over full force now.

"Fine. I went to visit my parents, they force me to do that once a month. Happy now?" Haru turned away, not wanting to look at Rin's ridiculous dumbstruck face. He'd just burned Rin, hard. It'll probably take Rin a week or two to recover from that burn.

" _Thank you very much."_ Rin tried to snarl back, Haru simply smirked. He had one this one. Rin couldn't hide the embarrassed red in his cheeks. Not wanting to deal with the apathetic freestyle swimmer anymore, Rin stalked out of the house, leaving Haru lying on the floor where he'd dumped him.

On to the next suspect.

This suspect had a higher probability of actually being the culprit. Nice guy, loads of muscles to feed Gou's muscle fetish, tall and hot, and overall perfect boyfriend. You know who we're talking about here. It's Tachibana Makoto.

Rin had been banging on the door for the past 3 minutes. There was only 1 pair of shoes outside the door, Makoto's usual orange sneakers. Meaning he was the only one at home, or he was too lazy to bring his shoes into the house after stepping in. Hmm, Makoto wasn't one to usually ignore people banging on his door. Meaning something was rather wrong here.

Not waiting to find out, Rin headed to the back door and tried to open that one. Locked again. Apparently, the Tachibanas value home security far more than a certain Nanase. Looking directly up, he could see a bedroom window open. And a few rocks very conveniently placed in the garden. Grabbing one, he hurled it into the open window. No sound. Taking another, he took another throw. It bounced off the side of the house with a loud CRACK, landing back on his head. It hit him on the forehead. Rubbing his forehead a little, he had to admit his aim wasn't that good. That's why he was on the swim team, not the basketball team. Grabbing another. Third time's the charm. Step back, aim high, and throw.

That did the trick. It seemed to have hit SOMETHING, because he heard a CRACK and someone shouting from inside. It sounded like curse words, but Rin couldn't tell. He didn't want to. Instead, he took a deep breath and shouted out. "OI MAKOTO! YOU IN THERE?"

He distinctively heard a loud grunt coming from inside, followed by heavy footsteps. Running back around to the door, he wanted to be ready to give this guy a good questioning/beating for whatever he'd done. Standing outside, he expected to see a usual cheery Makoto, smile and all. What he did not expect, was the teenage boy, looking groggy, _grumpy_ , hair all over the place, wearing an oversized t-shirt and shorts. Which is saying something, considering the size of Makoto. Where would he find clothes that were baggy on him? Anyways, back to the point.

"Rin. WHY are you here?" Now this was new. Always happy Makoto, is grumpy. He stared back at Rin with tired eyes, indicating lack of sleep. Oh right, he was here to ask something.

"Makoto, I'll be quick." Rin had to clear his throat, because honestly, he was still reeling from the sight of a rather different Makoto than the one he knew. "Where did you go… on Saturday?"

"To watch a movie." Geez, was Haru's bluntness starting to rub off on Makoto after all this while? Wait…. **Movie**?

"…. Who were you with?" Rin tried not to act too surprised. Haru had that leverage now, he didn't want to give it to Makoto too.

"…The little sister." Makoto's eyes were drooping, he had to stifle a yawn. He clearly wasn't interested in entertaining Rin at the time. Turning back into the house to leave, he wasn't prepared for the shove that came.

"WHY DID YOU GO OUT WITH GOU?!" Rin had just slammed Makoto to the wall of his own house. Simple question. Tone left to be desired. Whatever, culprit found.

"…What are you talking about? I didn't go out with Gou…" Makoto was looking even more confused now, not even trying to fight back against Rin's death grip on his neck. He was simply too tired to put up a fight.

"…Huh?" now for the second time today, Rin was feeling incredibly stupid. Who could blame him? Overprotective brother mind taking over logical mind, hello?

"I brought Ran to watch this new movie because my parents weren't free. Speaking of Gou, I did see her at the movie theatre with some guy." Rin started to loosen his grip on Makoto, leaning his face closer to the other's. _Now_ he's paying attention.

"What did the guy look like?" this could be his biggest hint all day, Tachibana had better not mess it up like he usually screws things up by being too wishy-washy. Seriously, his head was always in the clouds, how did he manage to take care of himself, much less Haru?

"I couldn't see, it was dark."

"Great help." Really. Very helpful information was provided right there. "What about after the movie?" he tried to sound nice, because he _was_ asking a favour of Makoto, least he could do was not be an ass about it. But the way Makoto was acting made it _really_ difficult to play nice. He just hoped the air-headed giant would get the hint and spit it out already.

"I'm not quite sure…" really? Right now? How does Makoto manage not flunking out of school? "He had a hoodie covering the back of his head… ah! It was a Samezuka swim team hoodie!" Makoto was snapping his fingers and beaming up like a child who'd just figured out how to do his homework. In all sense, it looked kind of cute, but Rin was in a rather foul mood so it just looked annoying to him.

"Thanks Makoto." Yes. Rin Matsuoka is thankful to someone for the help. It's not that big of a surprise. But the info he got, while vague, was the biggest hint he had all day. "I'll be going now. Go back to sleep or something." Rin brushed out the door, slamming it shut on his way out. Makoto looked sick, he didn't want to stick around and catch some nasty germs or something.

Rin was jogging back to Samezuka. The sun was beginning to set, he'd spent 3 hours running about interrogating his suspects. And he still hadn't found his culprit. _'Wearing a Samezuka swim team jacket.'_ That narrowed it down quite a bit. Who on the swim team had feelings for Gou? Then it hit him like a rock. How did he not notice until now? The one who sent him on this wild goose chase in the first place! Such an obvious distraction! That guy was childhood friends with Gou, nice to her while being a jerk to everyone else, it was so obvious! Yamazaki Sousuke had some explaining to do. Rin was going to be back to the dorm in 5 minutes, and he'd better be ready to give some explanations by then.

Rin had just sprinted all the way from Iwatobi's district 60 to district 40 in 10 minutes. (A/N: I don't know where Samezuka or Haru/Makoto's houses are. So I'll just number the districts like this to indicate the huge distance between.) Normally, it would take a 20 minute bus ride, but Rin was rather fit, and he had a major sense of urgency. He could run _really fast_ when he wanted to, he had the stamina for it too. Reaching the door of his shared room, he was huffing and panting. His search was almost over. Just past this door.

BANG! Rin just slammed open the door loud enough, he was sure that whole dorm block could hear the slamming of the hardwood door against the cement wall. He didn't care. He was on a mission, and said mission was about to come to an end. "SOUSUKE!" he was too focused on the person lying on the bed in front of him, he'd neglected to close the door after stepping into the room. What irked him more is the fact Sousuke had decided to use Rin's bed while he was out. What nerve! This guy has some real balls, he had to admit. Barely anyone dares to go up against an angry Matsuoka. _Especially_ when that Matsuoka is Rin.

Now Sousuke was looking rather surprised for someone who should've expected this coming. He'd just been grabbed by the front of his shirt, thrown off the bed and onto the hardwood floor of the dorm with a loud THUD. Rin, the person who'd obviously thrown him, stood over him in all his glory. Menacing gaze staring down to the helpless teen, he actually feared what Rin might do to him in his 'blind rage'. "Sousuke. **How dare you**." Rin was hissing. Soft (compared to before), calm, but filled with venom. Not to be mistaken for safety. RED DANGER ALERT. ANGRY MATSUOKA RIN ON THE LOOSE.

Sousuke could only look up at his best friend in shock, maybe a tinge of fear in his heart. Angry Rin was not something to be trifled with. "Rin… what are you?" he could barely mouth out the words before Rin stepped on his face. Literally. Slammed his entire right foot, shoe and all, into Sousuke's face. The dirt landed in Sousuke's mouth. It was disgusting. Rin had not bothered to remove his shoes when he entered the dorm.

" **How dare you**. Going out with Gou, and then tricking me into a wild goose chase?! YOU DID THIS FOR LAUGHS DIDN'T YOU?!"

Now it's true that Sousuke liked to do things just to see his friend's reaction, or see how dumb Rin could get. This time, however, he was genuinely confused. "Rin! What are you talking about? Get off me!" yup, Rin was _sitting_ on Sousuke's chest. And pressing a hand down on his _injured_ shoulder. If a stranger came through the open door at the moment, he might think they were having sex or something. Because Rin's legs were spread open, one on each side of Sousuke's chest, with poor Sousuke looking up at Rin in fear/horror/bewilderment. Rin was sitting on his diaphragm, making it harder to breathe by the second. He was already panting. As a swimmer, he could hold his breath, but not for so long. If he didn't shake Rin off soon, he'd faint from oxygen deprivation.

"Not until you tell me the truth!"

"WHAT TRUTH?! I DIDN'T DO ANYTHING!" Sousuke was trying desperately to hump his chest to force Rin off, but despite appearances, Rin was stronger than him. Probably due to his entire body breaking down from the brutal training over the years.

"You went on a date with Gou and didn't tell me, then you lied to me and sent me on a wild goose chase just for LAUGHS!" Rin had finally gotten off Sousuke's chest. Thank god. But instead, he was slammed into the nearest wall, with Rin gripping his neck tightly. At least he could breathe.

"I'm your best friend! You think I'll… do something like that?!" he managed to choke out, taking in very shallow breaths due to the vice grip on his throat. "Have you forgotten I was with you for the entire time on Saturday?!" with one surge of strength, Sousuke used his good arm to grip Rin's wrist, twisting it at an odd angle, effectively reversing their positions.

Now for the third time in the day, Rin felt incredibly dumb. This had to be the record for the number of times in one day he was made to feel stupid. Rin's back was pressed against the wall, Sousuke's eyes looming down on him dangerously. He suddenly felt guilty for accusing his best friend for something he had nothing to do with. "Sorry…" he tried to say.

"It's fine. It doesn't really matter."

"It matters! So who's the one who went out with Gou on Saturday?!" Sousuke had backed away, choosing to lie on the bottom bunk, but Rin didn't really care. He still had a mission to fulfil.

"You know, if you didn't run out of the dorm when I showed you that post, it would have saved you a whole lot of trouble." Sousuke pulled himself up from the bed, heading off to the computer. Rin was still standing frozen in spot from where Sousuke had left him in their little scuffle. Yup, this **had** to be the most number of times in a single day when he was left feeling like a complete idiot.

Sousuke had booted up the computer and called Rin over. They spent 5 minutes searching through his news feed until they could finally find the post from before. Needless to say, Sousuke was rather afraid of what would happen when he scrolled down to the picture. So as a warning, he spoke first. "Rin, sure you won't try to kill the guy in the picture?" knowing full well Rin's anger issues when it came to being a protective older brother.

"Yeah, yeah. I promise not to kill whoever went out with Gou on Saturday."

Not so satisfied, Sousuke didn't move. Rin was already very irritated from the day's events. He shoved Sousuke off the chair by his right side, making him land on his left shoulder, the uninjured one. Even in his irritation, Rin cared about his friend's safety. He grabbed the mouse, scrolling down to view the supposed picture of Gou and her 'date'. It couldn't have been more obvious. Why didn't he think of it before?!

"! MIKOSHIBAA!" his face was contorted with rage, tick marks forming all over his forehead.

"Oh? That's his name? I couldn't recognize him. Is he related to Momo?" Sousuke sat up on the floor, rubbing his left side where his giant ass had landed on.

"OH. HE'S MOMO'S BROTHER ALRIGHT." Rin seethed as he stormed out of the room, down the hallway to where Momotarou and Nitori's room was. If one person could figure out where Seijurro Mikoshiba had disappeared off to after graduation, Momo is the only one. After all, they seemed rather close when he saw the brothers bonding together in the occasions that Seijurro had come back to visit the Samezuka swim team.

He didn't even bother knocking. He just slammed open the door to the dorm room when he reached. "MOMO!" he looked around the seemingly empty room, just as messy as when he'd entered a few hours earlier.

"Oi Momo. Where are you? I need to ask you something important." Stepping into the room, he turned to the blind corner where the closet was. There stood a furiously sweating Momo and Nitori, backs pressed to the closet. "…. What are you guys doing?"

"U-uh! Nothing, Rin-senpai!" Nitori stuttered, which was perfectly normal, since he could hardly speak properly in the presence of his senpai.

"Anyways, Momo. I needed to ask you something important. Do you know where to find your brother? I need to speak to him urgently." Rin's voice was calm as ever, masking the deep anger he held within for the older Mikoshiba.

"I-I don't know…" Momo was really sweating here. His eyes darted over to the closet for a quick glance, hastily turning back to focus his gaze on Rin.

"Really. Why are you staring at the closet? Can I take a look inside?" Rin knew full well this was highly inappropriate, but with the way Momo was acting, something fishy was up, he just knew it.

"N-No! I keep my _personal_ stuff in there…" Momo screeched, throwing his arms up behind him, as if it would stop Rin from trying to open the closet.

' _Like porn? I wonder if Momo even owns porn…'_ this kid was about as predictable as a hurricane, which is to say not at all. One would never know what's up when he behaves weirdly. Shoving him aside, since Momo was smaller than him after all, Rin placed a hand on the wooden frame of the closet. It was shaking a little, you wouldn't be able to tell if you weren't in direct contact with the surface.

' _What the hell?'_ he steadied himself, throwing open the wooden doors as violently as he could. Out popped a huge man with orange-red hair. The one person he's looking for.

" _What the hell was that?!_ **Hello** **Captain** **Mikoshiba**." Rin snarled. "How nice of you to **drop in**." bad puns over there, Rin's not sorry.

"H-Hey Matsuoka…. What brings you here?" Mikoshiba looked up, trying his best to feign innocence. Judging by the look on Rin's face, he'd drastically failed.

"It just so happened I was **looking** for **you** …" Rin was glowering by now. Seijurro had stood up to face him at their full height, forgetting the fact that he was still in his little brother's dorm room, with another of his juniors.

"Okay. I'm here. What do you need me for?" he was trying to sound as calm as possible, sort of working this time around. He held up his hands in front of his chest defensively, knowing full well of Rin's rather short fuse.

"YOU SHOULD KNOW DAMN WELL WHY I WANT TO SEE YOU, **MIKOSHIBA**!" Rin hollered, grabbing the front of Seijurro's shirt, shoving his former captain against the nearest wall.

"WHOA WHOA! Calm down, Matsuoka! I-I can explain!" Seijurro was desperately pleading with the angry Rin, not really working since he was in a rather compromised position.

"Explain why you went out with Gou without telling me! **I would love to hear it**." Rin snarled, the same voice he used against Sousuke.

"It's a f-funny story… really…"

" **I'd love to hear it**." Rin's eyes narrowed a little, obviously hinting to poor Seijurro that he was losing his patience. "Out with it!"

"So uhhh.. I asked Gou-san out to catch this movie I wanted to watch. I wanted to go with Momo but he decided to leave me instead to go shopping." Seijurro sheepishly tried to explain, glaring over at his little brother who'd backed into the other corner of the room. "she agreed, and that's it."  
"Are you **sure** that's all there is to it?" Rin towered over the ex-captain, obviously not satisfied with the explanation.

"I swear! That's all there is to it!" Seijurro squeaked defensively. "I'll call Kou-chan right now if you don't believe me.." he said, reaching for his pocket.

Not wanting this creep to make any more contact with his beloved little sister, Rin snatched the phone out of Seijurro's hand and handed it to Momo. "Nope. Not gonna happen."

Handing it to Momo, the phone probably wouldn't have a good ending. Maybe it'll go into the pool like Rin's phone. Maybe Momo would look for porn in it. Rin didn't know. Rin didn't care.

"Get your ass ready, Mikoshiba. You're in for the beating of a lifetime." Rin cracked his knuckles, Seijurro gulped. Throwing him onto the floor, Rin raised one foot, and slammed it down onto Seijurro's face. Hard. And again. And jumped onto his chest too, for good measure. That went on for a good 5 minutes with his juniors cowering in the corner of the room. "Well, I'm done." Rin looked down to a bruised, suffering Seijurro. "You can go back to doing whatever you were doing. I don't care." Without another word, Rin slammed the door on his way out. His job was done. Culprit found, culprit beaten up, he was going to take a long deserved shower and get some SLEEP.

And that makes the daily life of protective older brother Rin Matsuoka.

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 **A/N: How was it? Funny enough? Please review. Writer is a little potato who lives off reader reviews :D**


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